A) Field of the Invention
The present invention relates to a solid state image pickup device, and more particularly to a solid state image pickup device to be used for a single plate image pickup apparatus.
B) Description of the Related Art
Charge coupled device (CCD) type and metal-oxide-semiconductor (MOS) type solid state image pickup devices are used as area image sensors of image pickup apparatuses such as video cameras and still image digital cameras.
CCD and MOS solid state image pickup devices have a number of photoelectric conversion elements disposed in and on the principal surface of a semiconductor substrate in a matrix shape having a plurality of rows and columns and generate output signals (pixel signals) from electric charges accumulated in these photoelectric conversion elements upon incidence of light. In many solid state image pickup devices, photoelectric conversion elements and an output signal generator unit for generating a pixel signal are integrated on the same semiconductor substrate.
The output signal generator can be classified into two types depending upon the structure. One type of the output signal generator transfers charges accumulated in photoelectric conversion elements to a charge detector circuit by using one or two kinds of charge transfer units constituted of CCDs and generates output signals at the charge detector circuit.
A CCD solid state image pickup device used as an area image sensor generally has a first charge transfer unit (hereinafter called a “vertical charge transfer unit”) provided for each photoelectric conversion column and a second charge transfer unit (hereinafter called a “horizontal charge transfer unit”; electrically connected to the vertical charge transfer units. CCDs can be made by forming a channel in a surface layer of a semiconductor substrate and forming a plurality of electrodes (transfer electrodes) on an electrically insulating film formed on the surface of the channel.
The other type of the output signal generator is a MOS solid state image pickup device. In this generator, a photoelectric conversion element is connected via a transistor to an output signal line, and an output signal is generated by forming a voltage or current signal on the output signal line corresponding to the charges accumulated in the photoelectric conversion element. The transistor is a switching element or the like which electrically connects the photoelectric conversion element and the output signal line at a predetermined timing.
A CCD or MOS solid state color image pickup device used by a single plate image pickup apparatus has generally a color filter array disposed above photoelectric conversion elements. This color filter array has color filters of a plurality of colors disposed in a predetermined pattern. One color filter is provided for each photoelectric conversion element. There are primary color filters and complementary color filters.
In order to increase the amount of incidence light upon each photoelectric conversion element, a micro lens array is disposed above the color filter array in many cases. This micro lens array has a number of micro lenses each disposed corresponding to each photoelectric conversion element.
In a conventional solid state image pickup device having a color filter array and a micro lens array, light fluxes having a variety of wavelengths converged by a micro lens become incident upon the underlying color filter. Light capable of entering each photoelectric conversion element is only the light having wavelengths in a predetermined range capable of transmitting through the color filter above the photoelectric conversion element, excluding stray light.
For example, if a solid state image pickup device has a primary color filter array of red, green and blue filters, light capable of being incident upon the photoelectric conversion element under the red filter is only red light in a predetermined wavelength range capable of transmitting through the red filter. Green and blue light entered the red filter cannot become incident upon the photoelectric conversion element under the red filter.
Each photoelectric conversion element generates charges corresponding to the amount of incident light. If the amount of incident light is small, the amount of charges accumulated in the photoelectric conversion element is also small.
Recent solid state image pickup devices have a high resolution and a high integration of photoelectric conversion elements. Micro lenses and underlying photoelectric conversion elements are becoming small. Even if a micro lens is used, the amount of incident light upon a small photoelectric conversion element decreases and the sensitivity of the solid state image pickup device is likely to be lowered.